Conventionally, liquid fuel is injected into an internal combustion engine, for example a compression-ignition engine, using a means that establishes fuel pressure, which is in the form of an injection pump capable cyclically of placing fuel under high pressure in a variable-volume chamber formed in a pump cylinder and delimited by a plunger actuated by a camshaft that is kinematically connected to the main shaft or crankshaft of the engine.
The compressed fuel is injected through an injection nozzle which communicates, via a number of injection holes with the working or combustion chamber of the engine, this nozzle including a chamber which communicates with the cylinder of the injection pump, generally through one-way passage means such as a non-return valve. The communication between this chamber and the working or combustion chamber is interrupted by a sliding needle resting, by virtue of the action of elastic return means, on a conical seat formed in the said nozzle upstream of the said injection holes.
This needle includes a cylindrical body sliding in a bore formed in the nozzle and of a diameter that exceeds that of the conical bearing surface of the needle on its seat, so that when the pressure of the fuel exceeds a certain value, known as the rated pressure of the injector, it lifts off its seat to allow injection to take place. When the plunger of the injection pump has travelled beyond a certain position, beyond which the pump cylinder will find itself in communication with low-pressure fuel-supply means, the pressure in the nozzle drops and the injection needle is returned onto its seat, to close the nozzle again, under the action of the aforementioned elastic return means.
In spite of its great simplicity, this conventional device presents serious drawbacks. Firstly, the injection pressure depends on the engine speed and varies with it. Next, because of the necessarily limited force of the return spring, the end of injection occurs at a low pressure which no longer allows the fuel to be finely atomized, this generating unburnt hydrocarbons and particles of soot.
Some of these drawbacks may be avoided using an injection device of the type proposed in Patent FR-A-1,351,593. In such a device, fuel from the pressure-establishing means, such as a pump or an accumulator, is sent to an injection chamber delimited by a plunger moving against a powerful spring, so as to fill the chamber with pressurized fuel during the non-injection phase. This chamber opens directly into the injector cavity in which the shut-off means, such as a needle whose lift can be controlled either by an independent hydraulic circuit or by a circuit using fuel, move. When the needle is made to open and therefore injection is made to take place, the needle moves, over a long travel, until it comes into abutment against the plunger of the injection chamber, this plunger, as it advances delivering fuel from the injection chamber to the injection nozzle, causing the needle to close again.
Such a device does, however, have drawbacks which make it unsuitable for use with very high injection pressures which are used in modern engines and which would result in a heightened risk of fuel leaks and in a loss of power because of some of the fuel spilling out of the injector volume into the supply circuit, not to mention the problem of over-specifying the return spring.
DOS 1,944,862 has also already described an injection device comprising an injection chamber in which a differential plunger moves and which is in direct communication with the injector, control means allowing the injection chamber to be filled, during the non-injection phase, against the action of the pressure of a fuel accumulator kept charged by a high-pressure pump. This device does, however, present appreciable drawbacks. In particular, the injection pressure is only a fraction of the pressure supplied by the pump and the end-of-injection-travel of the plunger is marked by a pressure drop that is difficult to obtain precisely, particularly in the case of very high pressures.
In order to overcome the drawbacks of these various solutions, modern developments have concentrated on accumulation-type injection systems, also known as "common rail" systems.
In such systems, an accumulator is constantly filled with fuel at high pressure by virtue of means that generate high pressure in the fuel, and permanently communicates with the chamber of the injection nozzle upstream of the seat against which, by virtue of control means, the bearing surface of the injection needle presses. A distributor allows the said chamber to be made to communicate with another chamber delimited by the upper face of the needle, so that the needle is pressed against its seat under the effect of the pressure prevailing in the accumulator. When injection is to be initiated, the distributor is switched to make the said chamber delimited by the upper face of the needle communicate with low-pressure feed means so as to make the pressure exerted on the upper face of the needle drop so that the needle can lift.
To end the injection, the distributor is switched again into the other position, so as to re-establish the accumulator pressure over the upper face of the needle, so that the control means return the needle onto its seat without it being necessary for the injection pressure to be dropped. Injection therefore takes place entirely at the high pressure of the accumulator, thus avoiding drips at the end of injection.
Furthermore, the injection pressure is independent of the engine speed.
Such devices, which are advantageous from the points of view of combustion quality and control of unburnt hydrocarbons, do, however, present other drawbacks. Specifically, if the needle does not close properly or if the needle seat is damaged, the fuel present at high pressure in the accumulator will spill out into the combustion chamber, with a risk of the engine becoming overheated and destroyed.
Furthermore, as the permanent pressure of the common rail is sealed in by numerous plungers sliding in bores, a significant level of leakage creates mechanical losses, heating of the fuel and disturbs metering accuracy.